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Sampling to obtain signal from rfid card

Sampling to obtain signal from rfid card description/claims

The radio frequency identification (RFID) industry employs a number of frequencies and modulation techniques to communicate information between cards and readers. The reader comprises a crystal and additional electronics that serve to generate a carrier signal of usually one hundred twenty-five (125) kHz. The access control portion of the industry has historically employed the carrier signal frequency to power passive credit-card-size cards that communicate codes back to the reader. The magnetic field generated by the carrier signal frequency serves to power, at a range of a few inches or several centimeters, the credit-card-size cards that comprise a coil resonated by a capacitor and electronics such as an integrated circuit (IC) powered and clocked by the magnetic field. The electronics of the card responds to the magnetic field and loads coded information from the card onto the magnetic field in a time dependent manner that is determined by the card electronics. The card communicates a unique serial code of usually thirty-two (32) to two hundred fifty-six (256) bits back to the reader to establish the identity of the holder or owner of the card.

The card employs modulation techniques that depend on the particular manufacturer of the equipment. The various manufacturers of access control equipment have chosen differing modulation techniques to transfer information in a bit stream from the cards to the reader. Common modulation techniques employed with access control cards comprise: direct modulation of the carrier signal in amplitude-shift keying (ASK); frequency-shift keying (FSK) modulation of a sub-carrier signal generated by the electronics of the card; or phase-shift keying (PSK) modulation of a sub-carrier signal generated by the electronics of the card. The electronics of the card typically employs digital division of the carrier signal frequency to generate the bit rates and sub-carrier signal frequencies. The electronics of the card usually imparts additional encoding and bits to the serial code such as Manchester and/or bi-phase encoding, and/or parity bits and/or check sums, to provide a serial card data stream as an output.

Features of exemplary implementations of the invention will become apparent from the description, the claims, and the accompanying drawings in which:

FIG. 1 is a representation of an implementation of an apparatus that comprises a radio frequency identification (RFID) reader, an RFID tag and/or card, a card-reader interface, and/or a readout device.

FIGS. 2 and 3 represent an exemplary logic flow that may be performed by the RFID reader for searching for one of a plurality of possible signal formats of the RFID card of an implementation of the apparatus of FIG. 1.

Referring to the BACKGROUND section above, a typical access control reader employs a magnetic field generator coupled with a peak detector that serves to detect modulation of the magnetic field by the card. The output of the detector is amplified and then digitized by a Schmitt trigger. The digitized signal is then decoded by a processor, for example, a microprocessor, and data is transferred out of the reader, often in a security card data encoding format such as Wiegand format, for example, the standard 26-bit format. A consideration for the circuit designer is that the modulation can be very small compared to the carrier signal amplitude. So, the signal processing may involve enhancement of the modulation signal and rejection of the carrier signal frequency of usually one hundred twenty-five (125) kHz. An implementation may relatively easily handle amplitude-shift keying (ASK) signals, but may have progressively more difficulty with frequency-shift keying (FSK) signals and phase-shift keying (PSK) signals, for example, as the frequencies of interest are closer to the carrier signal frequency and the detected signal is smaller in amplitude relative to the carrier signal amplitude. In a case of PSK, the sub-carrier signal frequency may comprise a relatively small separation such as only an octave below the carrier signal and the recovered signal may be very small, for example, due to a typically high Q factor of an antenna coil of the reader magnetic field generator. A high Q factor slows the response of the reader antenna amplitude, which accounts for the relatively low level of response to PSK signals at one octave below the carrier signal frequency.

The direct modulation of the carrier signal in ASK usually occurs at approximately two (2) or four (4) kHz, derived by dividing the carrier signal frequency by sixty-four (64) or thirty-two (32). The FSK modulation of a sub-carrier signal usually occurs by the data stream switching the sub-carrier signal between 12.5 kHz and 15.125 kHz, derived by dividing the carrier signal frequency by ten (10) or eight (8). Typical PSK modulation of a sub-carrier signal occurs from influence of the serial data stream upon the sub-carrier signal, derived by dividing the carrier signal by two (2) and phase shifting the carrier signal usually by one hundred eighty (180) degrees.

The various possible modulation techniques present a consideration for manufacturers of reader equipment that may need to receive any of the modulation techniques. For example, computers can be equipped with card readers to control access to their use. A manufacturer of the computer could need to incorporate different readers for each of the possible modulation techniques and card data stream formats.

Multiple signal paths have been employed for the different modulation techniques. The reading of multiple formats of low frequency passive radio frequency identification (RFID) tags can operate by first determining the nature of the modulation being received and then directing the received signal through an amplifier specific to the modulation bandwidth. Various techniques have served to determine or direct the path that corresponds to a particular modulation scheme such as passive filtering, active processor determination, time-share multiplexing, or user-determination either manually or by computer control. The different modulation techniques can lead to complex circuits, for example, with different portions used for the various modulation techniques. The requirement of the separate signal paths for the various modulation techniques can add complexity and cost.

An exemplary approach deals with ASK, FSK, and PSK all in a signal path design. An exemplary implementation comprises a multi-mode RFID Reader. An exemplary implementation employs a synchronous and/or substantially synchronous sampling technique that allows a reader to read access control cards from a variety of manufacturers with a relatively simple and/or inexpensive electronic circuit. An exemplary implementation employs sampling that allows ASK, FSK, and PSK modulation techniques to pass a usable signal to a processor for reducing that signal to an identification (ID) number. In an exemplary implementation, sampling synchronous and/or substantially synchronous with the reader excitation field provides a desired rejection of the carrier signal frequency. In an exemplary implementation, the signal coming from the card is synchronous and/or substantially synchronous with the magnetic field generated by the reader to allow the card data stream to be recovered with a simple reader circuit regardless of the modulation technique employed.

Turning to FIG. 1, an implementation of an apparatus 100 in an example comprises a radio frequency identification (RFID) reader 102, an RFID tag and/or card 104, a card-reader interface 105, and/or a readout device 106. The card-reader interface 105 in an example comprises a magnetic field generated by an antenna 114 of the RFID reader 102. The RFID card 104 in an example loads the magnetic field in a predetermined way, for example, through employment of a predetermined card data stream to provide information on a carrier signal. The RFID card 104 in an example comprises a passive, proximity, and/or contactless card. As described herein, electronics of the RFID card 104 in an example responds to the magnetic field and loads coded information from the RFID card 104 onto the magnetic field in a time dependent manner that is determined by the electronics. The RFID card 104 in an example communicates a unique serial code, for example, of thirty-two (32) to two hundred fifty-six (256) bits, back to the RFID reader 102 to establish an identity of a holder or owner of the RFID card 104.

The RFID reader 102 in an example comprises a crystal 107, a clock oscillator 108, a binary counter 110, an antenna driver 112, an antenna 114, a capacitor 116, an amplitude detector 118, a sample and hold circuit 120, a bandpass amplifier 122, a sample and hold circuit 124, a bandpass amplifier 126, a sample and hold circuit 128, a Schmitt trigger 130, a sample and hold circuit 132, a processor 134, and an output interface 136.

The crystal 107 and the clock oscillator 108 in an example cooperate to produce a frequency 138 output from the clock oscillator 108 for input to the binary counter 110. The frequency 138 in an example may be selected to comprise a substantially binary or power-of-two multiple of an RFID industry standard frequency, for example, a substantially binary multiple and/or power of two multiple of one hundred twenty-five (125) kHz that may be selected in view of an exemplary guide and/or standard for the RFID card 104. The binary counter 110 in an example comprises the frequency 138 that is input to the binary counter 110 from the clock oscillator 108 and a plurality of frequencies 140, 142 that are output from the binary counter 110. The binary counter 110 in an example outputs the frequencies 140, 142 as substantially square waves.

The binary counter 110 in an example comprises a plurality of stages, for example, arranged in series. Each stage of the binary counter 110 in an example serves to divide by two (2) the frequency input to that stage, such as from a preceding stage or, such as at the initial stage, directly from the clock oscillator 108. An exemplary input of eight (8) MHz from the clock oscillator 108 to a binary counter 110 that comprises a plurality of stages in an example comprises a number of outputs such as any of four (4) MHz, two (2) MHz, one (1) MHz, five hundred (500) kHz, two hundred fifty (250) kHz, one hundred twenty-five (125) kHz, sixty-two and one-half (62.5) kHz, and so on as may be desired and/or selected.

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• Utility Patent

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